diff --git a/CHANGELOG.md b/CHANGELOG.md
index de60100..be37493 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ### Unreleased
 
- * Put diff here
+ * Allow CSS functions to be used in CssParser::RuleSet#expand_dimensions_shorthand! [#126](https://github.com/premailer/css_parser/pull/126)
 
 ### Version 1.9.0
 
diff --git a/lib/css_parser/regexps.rb b/lib/css_parser/regexps.rb
index 9b3bcf6..a76ffda 100644
--- a/lib/css_parser/regexps.rb
+++ b/lib/css_parser/regexps.rb
@@ -58,6 +58,22 @@ def self.regex_possible_values(*values)
   RE_BORDER_STYLE = /(\s*^)?(none|hidden|dotted|dashed|solid|double|dot-dash|dot-dot-dash|wave|groove|ridge|inset|outset)(\s*$)?/imx.freeze
   RE_BORDER_UNITS = Regexp.union(BOX_MODEL_UNITS_RX, /(thin|medium|thick)/i)
 
+  # Functions like calc, var, clamp, etc.
+  RE_FUNCTIONS = /
+    (
+      [a-z0-9-]+        # function name
+    )
+    (?>
+      \(                # opening parenthesis
+        (?:
+          ([^()]+)
+          |             # recursion via subexpression
+          \g<0>
+        )*
+      \)                # closing parenthesis
+    )
+  /imx.freeze
+
   # Patterns for specificity calculations
   NON_ID_ATTRIBUTES_AND_PSEUDO_CLASSES_RX_NC = /
     (?:\.\w+)                     # classes
diff --git a/lib/css_parser/rule_set.rb b/lib/css_parser/rule_set.rb
index 793e80f..bd880a8 100644
--- a/lib/css_parser/rule_set.rb
+++ b/lib/css_parser/rule_set.rb
@@ -24,6 +24,8 @@ class RuleSet
       ['border-width', %w[border-top-width border-right-width border-bottom-width border-left-width]]
     ].freeze
 
+    WHITESPACE_REPLACEMENT = '___SPACE___'
+
     class Declarations
       class Value
         attr_reader :value
@@ -358,7 +360,7 @@ def expand_dimensions_shorthand! # :nodoc:
         # TODO: rgba, hsl, hsla
         value.gsub!(RE_COLOUR) { |c| c.gsub(/(\s*,\s*)/, ',') }
 
-        matches = value.strip.split(/\s+/)
+        matches = split_value_preserving_function_whitespace(value)
 
         case matches.length
         when 1
@@ -374,8 +376,7 @@ def expand_dimensions_shorthand! # :nodoc:
           raise ArgumentError, "Cannot parse #{value}"
         end
 
-        t, r, b, l = values
-        replacement = {top => t, right => r, bottom => b, left => l}
+        replacement = [top, right, bottom, left].zip(values).to_h
 
         declarations.replace_declaration!(property, replacement, preserve_importance: true)
       end
@@ -634,6 +635,19 @@ def parse_selectors!(selectors) # :nodoc:
         s
       end
     end
+
+    def split_value_preserving_function_whitespace(value)
+      split_value = value.gsub(RE_FUNCTIONS) do |c|
+        c.gsub!(/\s+/, WHITESPACE_REPLACEMENT)
+        c
+      end
+
+      matches = split_value.strip.split(/\s+/)
+
+      matches.each do |c|
+        c.gsub!(WHITESPACE_REPLACEMENT, ' ')
+      end
+    end
   end
 
   class OffsetAwareRuleSet < RuleSet
diff --git a/test/test_rule_set_expanding_shorthand.rb b/test/test_rule_set_expanding_shorthand.rb
index 2d41018..792285e 100644
--- a/test/test_rule_set_expanding_shorthand.rb
+++ b/test/test_rule_set_expanding_shorthand.rb
@@ -296,6 +296,54 @@ def test_expanding_important_shorthand_with_replaced_properties
     assert_equal expected_declarations, declarations
   end
 
+  def test_functions_with_many_spaces
+    shorthand = 'margin: calc(1em / 4 * var(--foo));'
+    declarations = expand_declarations(shorthand)
+    expected_declarations = {
+      'margin-top' => 'calc(1em / 4 * var(--foo))',
+      'margin-bottom' => 'calc(1em / 4 * var(--foo))',
+      'margin-left' => 'calc(1em / 4 * var(--foo))',
+      'margin-right' => 'calc(1em / 4 * var(--foo))'
+    }
+    assert_equal expected_declarations, declarations
+  end
+
+  def test_functions_with_no_spaces
+    shorthand = 'margin: calc(1em/4*4);'
+    declarations = expand_declarations(shorthand)
+    expected_declarations = {
+      'margin-top' => 'calc(1em/4*4)',
+      'margin-bottom' => 'calc(1em/4*4)',
+      'margin-left' => 'calc(1em/4*4)',
+      'margin-right' => 'calc(1em/4*4)'
+    }
+    assert_equal expected_declarations, declarations
+  end
+
+  def test_functions_with_one_space
+    shorthand = 'margin: calc(1em /4);'
+    declarations = expand_declarations(shorthand)
+    expected_declarations = {
+      'margin-top' => 'calc(1em /4)',
+      'margin-bottom' => 'calc(1em /4)',
+      'margin-left' => 'calc(1em /4)',
+      'margin-right' => 'calc(1em /4)'
+    }
+    assert_equal expected_declarations, declarations
+  end
+
+  def test_functions_with_commas
+    shorthand = 'margin: clamp(1rem, 2.5vw, 2rem)'
+    declarations = expand_declarations(shorthand)
+    expected_declarations = {
+      'margin-top' => 'clamp(1rem, 2.5vw, 2rem)',
+      'margin-bottom' => 'clamp(1rem, 2.5vw, 2rem)',
+      'margin-left' => 'clamp(1rem, 2.5vw, 2rem)',
+      'margin-right' => 'clamp(1rem, 2.5vw, 2rem)'
+    }
+    assert_equal expected_declarations, declarations
+  end
+
 protected
 
   def expand_declarations(declarations)